In recent years, the improvements of saving silver amounts used in silver halide photographic light sensitive materials (hereinafter referred to as light sensitive materials) have been rapidly progressed so as to make image quality higher and to save resources. With the purpose of satisfying the requirements for the above-mentioned improvements, a variety of studies on how to make silver halide grains smaller in size, how to improve aspect ratios higher and how to use monodisperse type twinned-crystals have been made from the viewpoints of silver halide emulsion preparation processes.
The techniques concerning silver halide grains having high aspect ratios are disclosed in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as "JP OPI Publication") Nos. 58-113926/1983, 58-113927/1983, 58-113928/1983 and 62-163048/1987. The silver halide grains disclosed in the above-given patents are the tabular-shaped grains having an aspect ratio of not lower than 8.
When a twinned-crystal grain having two or more parallel twin planes is projected from the direction vertical to the twinned-crystal faces, the term, `aspect ratio`, herein stated can be expressed by a ratio of a circular-equivalent grain diameter to a distance (i.e., a thickness) between the two parallel outer-surfaces of the grain; the circular-equivalent diameter is obtained by converting the area of the grain into a circular area.
When a silver halide grain can have a high aspect ratio, the following advantages can be enjoyed. The surface area per volume can be widened so that sensitizing dyes can be adsorbed more to the surface of the silver halide grain, as compared to the so-called regular-crystal silver halide grains such as those of a octahedron, tetradecahedron or hexahedron and, particularly in an X-ray light sensitive material, the light-absorption coefficient of a sensitizing dye is larger than the indirect-transition light- absorption coefficient of the X-ray light sensitive material so that the cross-over light, that is a characteristic of X-ray light sensitive materials, can remarkably be so reduced as to provide a higher sharpness.
In order to make an image-quality higher and a silver amount used smaller in a light sensitive material, it is required to make silver halide grains smaller in grain size. The term, `a grain size`, stated herein means a diameter obtained when the projective image area of a grain is converted into a circular area having the same area as that of the projective image. In the above-described process for preparing the grains, any silver halide grains each having a small grain-size and a high monodispersibility could not be prepared, though those having a high aspect ratio could be prepared.
Heretofore, there have been the patents disclosing monodisperse type twinned-crystal emulsions, such as JP OPI Publication Nos. 61-6643/1986 and 61-14636/1986. These patents disclose the techniques for preparing monodisperse type globular-shaped twinned-crystal emulsions prepared by applying an Ostwald ripening treatment thereto after producing the nuclei thereof and, serving the resulting emulsion as the seed emulsion, the seed emulsion is grown further, so that the monodisperse type twinned-crystal emulsion can be prepared. As compared to a polydisperse type emulsion mixed therein large-sized and small-sized grains together, the resulting monodisperse type twinned-crystal emulsion has such an advantage that is readily capable of applying an optimum chemical sensitization.
When making use of these seed emulsion, however, any silver halide grains having a small-size and a high aspect ratio could not be prepared, though highly monodispersed silver halide grains could be prepared.
Besides the above-described process for preparing the emulsions, on the other hand, there have been some similar techniques such as those disclosed in, for example, JP OPI Publication Nos. 1-158426/1989, 1-213637/1989 and 2-838/1990, in which a monodispersed twinned-crystal emulsion is prepared by providing an Ostwald ripening step after producing the nuclei thereof. In these similar techniques, a low molecular-weight gelatin is used when the nuclei is produced. However, such a low molecular-weight gelatin is non-advantageous as a protective colloid formed therewith will cause grain coagulation and sizes of the grains may seriously varied by a coalescence phenomenon produced between the grains so that the production stability may be disturbed thereby. In addition, the gelatin has another defect that the cost thereof is higher than that of the gelatin commonly used in preparing a silver halide photographic emulsion.
Example 1 given in JP OPI Publication No. 2-166442/1990 discloses a technique for producing tabular-shaped grains, in which a reaction vessel and a mixing vessel are so provided that silver halide fine-grains are produced by making use of a low molecular-weight gelatin and a dispersion medium that is a synthetic macro-molecular compound in the mixing vessel and, on the other hand, a twinned-crystal seed emulsion is dissolved in advance with gelatin in the reaction vessel and thereto the silver halide fine-grains are added from the mixing vessel so as to dissolve the fine-grains, so that the seed emulsion is grown up. However, this preparation process is to make smaller the grain sizes of the silver halide grains so that the grains may readily be dissolved, but not to make smaller the grain sizes of the resulting tabular-shaped grains.
Example 1 given in JP OPI Publication No. 2-28638/1990 discloses a technique in which a small grain sized, high aspect ratio, twinned-crystal silver halide emulsion is prepared in a ripening treatment carried out by raising a temperature to 65.degree. C. after producing the nuclei thereof at 30.degree. C. so that an average grain-size of 0.52 .mu.m and an average aspect ratio of 9.5 can be obtained. However, the resulting emulsion has a high variation-coefficient such as 30%.
The present inventors have tried to carry out an Ostwald ripening treatment under the presence of a silver halide solvent at a temperature higher than the temperature of producing the nuclei as disclosed in the above-given JP OPI Publications. However, the same monodisperse type twinned-crystal emulsion as that disclosed in JP OPI Publication No. 61-6643/1986 could not be prepared.